As regards the heat treatment of liquid or pumpable foods, use may be made of steam in order to heat the food product in a rapid and efficient manner. One of the methods which is used to inject steam direct into the product is by means of an injector.
Depending upon the temperature to which it is desired to heat the product, it is possible to obtain a pasteurized or an aseptic product, or alternatively a product with extended shelf-life in cold storage. The commonest practice is to heat the product, which may, for example, be dairy produce, juices, viscous food products or the like, to a temperature where a total extermination of harmful microorganisms is obtained. This gives a food product which maintains its stability on storage at room temperature. No unbroken refrigerated chain for storage is necessary, which may be an advantage above all in the developing countries.
Direct heating of the product by injecting steam into the product gives a rapid and efficient heating. As a result of this rapid method, it is possible to reduce the treatment time, which in total gives a reduced thermal effect on the product and a product will be obtained which maintains a higher quality, above all in terms of flavour.
There are a large number of injectors available on the market which all share a similar construction, with an inlet for the product which is to be treated and an outlet for the ready-treated product. The injector further displays an inlet for steam, under high pressure, which by various arrangements with gaps or channels is caused to converge with the product and which heats it to the desired temperature.
Swedish Patent Specification SE 367 121 describes a so-called annular gap injector, where concentric gaps for steam and product are disposed about a centre shaft. The annular gap injectors are known for their operational reliability but may occasionally become choked because of product burning onto surfaces, so-called fouling. In order to obviate this problem, the centre shaft has been extended, which often may be a sufficient measure. But on occasions the hot steam succeeds in passing through the colder layer of product and fouling nevertheless occurs.